The present invention relates to a laser lap welding method, and more particularly to a laser lap welding method which improves a hole, an indentation, and the like, which are caused at a welding terminating end.
A laser welding method, in which a laser beam is irradiated onto a workpiece to heat and melt a material of the irradiated portion by the light energy of the laser beam, has an advantage in that high speed welding can be performed in a non-contact manner, but has a problem in that a hole and an indentation are caused at a welding terminating end. Thus, the problem has become one of the factors that limit the use of the laser welding method to only some automobile parts and prevents the laser welding method from being used for into the vehicle body welding process in which strict management of performance and quality about airtightness, water leakage, and the like, is required.
Perforation and indentation, which are caused in a laser welding terminating end, are caused by molten metal supplied to the welding terminating end eventually becoming insufficient due to a phenomenon in which the molten metal flows in the direction opposite to the welding advancing direction. As a measure to solve this problem, there is known, as disclosed in JP2007-313544A, a method which is referred to as “ramping” or “fade down” and in which the laser output is controlled to be gradually reduced toward the welding terminating end.
For example, as shown in FIG. 5(A) and FIG. 5(B), in the case in which two galvanized steel sheets 1 and 2 are overlapped and laser-welded to each other, when the laser output P is maintained at a constant level until the laser beam reaches the welding terminating end, a hole 52 is generated at the end of a weld bead 51, and the substantial welding length Wa becomes shorter than the laser irradiation length L by the length corresponding to the hole 52.
On the other hand, as shown by the solid line (61) in FIG. 5(C) and FIG. 5(D), when the laser output P is gradually reduced toward the welding terminating end, since the penetration depth is gradually reduced, the frequency of occurrence of the perforation at the end of the weld bead 61 is reduced, but the perforation cannot be completely prevented. Furthermore, even in the case in which the perforation is not caused, a comparatively deep indentation 62 is left at the welding terminating end, and also the substantial welding length Wa′ is further reduced. Thus, when this welding method is used as it is, a reduction in strength, and the like, is caused at the welding terminating end, so that the welding quality is inevitably affected.
In order to avoid this problem, it is also conceivable to increase the welding length (L″) as shown by a broken line (71) in FIG. 5(C) and FIG. 5(D). However, in this case, problems are caused in which a space required for the weld bead 71 and the cycle time are increased.
As another measure against the above-described problems, a method is disclosed in JP2008-264793A in which the laser irradiation diameter is increased at the welding terminating end by defocusing the laser beam. However, as shown in FIG. 1 of JP2008-264793A, when the laser beam is stopped and defocused at welding terminating end, burn-through may be caused instead of an improvement in the hole and indentation. Furthermore, although not clearly described in JP2008-264793A, there arises a problem that, when the defocusing operation is started just before the laser beam reaches the welding terminating end, the energy density of the laser beam is reduced and thereby the substantial welding length is reduced similarly to the case in which the above-described method is used.